Feature: allow webpack 4 type javascript/esm

[hlehmann]

In webpack 4 we can set file types such as "javascript/esm" which have more restriction.

With type "javascript/esm" we can't use require, exports and module anymore. But there is still import, export and __webpack_module__

Could it be possible to add a esm param to the babel plugin to be able to only use esm import/export

// .babelrc
{
  "plugins": [["react-hot-loader/babel",{esm:true}]]
}

[theKashey]

There is no need for a special flag, but few things had to be rethinked.

• we could(and have to) remove module guards(enter/leave), as long all imports have to be in one place.
• do something with hot(module) - for example magically transform into hot(__webpack_module__ || module)